Septic shock with multiple organ failure is the leading cause of death in non-coronary intensive care units and remains a major health problem in the US. Endothelial cell dysfunction is an important hallmark of septic shock and results in increases in vascular permeability that play critical roles in the pathogenesis of multi-organ failure. Thus, improving endothelial function represents an important potential therapeutic target in sepsis. The long-term goal of our research is to identify novel treatment strategies for the endothelial injury of sepsis. The overall objective for this proposal is to identify the mechanisms by which endothelial progenitor cells (EPCs) modulate endothelial dysfunction in sepsis. EPCs are circulating, bone marrow-derived endothelial precursor cells which have been recently associated with sepsis. Specifically, EPCs from humans with sepsis demonstrate alterations in function compared to healthy controls and EPC proliferation is inversely related to organ dysfunction in human sepsis. Our data demonstrate that treatment with exogenous human cord blood derived EPCs improves survival, attenuates pulmonary vascular leak and improves organ injury in the murine cecal ligation and puncture (CLP) model of polymicrobial sepsis. A better understanding of the mechanisms by which EPCs exert their protective effects is crucial to the design of new therapeutic strategies. Endothelial cells (ECs) and EPCs are known to release exosomes and microparticles containing micro RNAs (miRNAs) that regulate endothelial function and barrier integrity. We have shown that treatment with EPCs alters circulating levels of EC relevant miRNAs in murine sepsis, including miR-126. Our findings provide strong evidence that miR-126 may play an important role in maintaining EPC/EC function and barrier integrity in sepsis. We propose two related hypotheses: 1) Endothelial progenitor cells regulate endothelial function and barrier integrity through release of specific miRNAs in sepsis and 2) Altered expression of miRNAs which regulate endothelial barrier integrity are associated with severity of illness and outcomes in human sepsis. Two specific aims address these hypotheses: Aim 1: Determine the mechanisms by which EPCs affect vascular barrier integrity in murine sepsis. Aim 2: Determine the expression patterns of miRNAs known to modulate endothelial function and barrier integrity in human sepsis. The proposed research will lay a critical foundation for the design of novel therapeutic strategies for sepsis.